combining materials

My question how can I put 2 materials into plane.
I’m trying to make an ocean plane and I need the outline to be different material for the shores. I don’t know if you can understand me, I’m quite new to the engine.
here’s and example of what I want it to be:

Vertex painting. Masks. Assigning multiple materials in your 3d application before importing. There’s a lot of ways to approach it.